In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography was stopped and instead, the ArF immersion lithography was introduced.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water having a refractive index of 1.44. The partial fill system is compliant with high-speed scanning and when combined with a lens having a NA of 1.3, enables mass production of 45-nm node devices.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line-edge roughness (LER or LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
Another candidate for the 32-nm node lithography is high refractive index liquid immersion lithography. The development of this technology was stopped because LUAG, a high refractive index lens candidate had a low transmittance and the refractive index of liquid did not reach the goal of 1.8.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern features. A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
The double patterning process suffers from problems including a failure to insure an overlay accuracy between two exposures, a reduction of throughput by two exposures, the complexity of pattern division and an increase of mask cost due to use of two masks.
In a sidewall spacer process, a resist pattern as developed is transferred to a hard mask, a silicon oxide film is deposited on sidewalls of the hard mask, and two lines are formed on sidewalls of each line. This can double resolution despite a single exposure. The sidewall spacer process overcomes the aforementioned problems of double patterning since only one exposure is necessary. However, the desired pattern is formed by repeating CVD and etching for deposition of silicon oxide film by CVD and removal of unnecessary silicon oxide film on lines and in spaces, resulting in a complex long process. The increased cost of extra steps other than the lithography is a problem.
Most cost effective is a technology capable of doubling the resolution of a resist pattern through a single exposure and development.
JP 3943741 discloses a pattern forming method capable of achieving a double resolution by using a hydroxystyrene based KrF resist composition and effecting two developments, organic solvent development and alkaline water development, thereby dividing one line into two lines. As the ArF resist composition for a combination of negative tone development with organic solvent and positive tone development with alkaline water, ArF resist compositions of the prior art design may be used as described in JP-A 2008-281974, 2008-281975, 2008-281980, 2009-053657, 2009-025707, and 2009-025723. An example of using an ArF resist composition and dividing one line into two by the above method is described in Non-Patent Document 1 (Proc. SPIE Vol. 6923 p69230F-1).
These patent documents disclose resist compositions for organic solvent development comprising a copolymer of hydroxyadamantane methacrylate, a copolymer of norbornane lactone methacrylate, a copolymer of methacrylate having acidic groups including carboxyl, sulfo, phenol, thiol and other groups substituted with two or more acid labile groups, and a copolymer of methacrylate having a cyclic acid-stable group ester, and pattern forming processes using the same.
The formation of negative pattern through organic solvent development is a traditional technique. A resist material comprising cyclized rubber is developed using an alkene such as xylene as the developer. An early chemically amplified resist composition comprising poly(t-butoxycarbonyl-oxystyrene) is developed with anisole as the developer to form a negative pattern.
In JP 4445860, a negative tone pattern is formed by EB image writing on calix-arene and developing with n-butyl acetate or ethyl lactate.
When a prior art photoresist composition is subject to alkaline water development and organic solvent development, the resolution can be doubled as described in Non-Patent Document 1. However, it is difficult to gain a resolution surpassing the maximum resolution that is available with a single exposure in an ordinary process.
In the case of an ArF resist composition of the prior art design based on a polymer comprising recurring units in which an acidic group such as carboxyl group is substituted with an acid labile group, deprotection of the protective group leads to improved dissolution in alkaline developer and reduced dissolution in an organic solvent developer. If the exposure dose at which alkaline dissolution is improved is coincident with the exposure dose at which the resist becomes insoluble in organic solvent, then a fine size pattern cannot be formed because there is no margin or, if any, a quite narrow margin for the exposure dose at which the resist is insoluble in both alkali and organic solvent. It would be desirable to have a resist composition wherein the exposure dose at which alkaline dissolution is improved is different from the exposure dose at which the resist becomes insoluble in organic solvent.